1. Field
Embodiments of the invention relate to electrolytic manganese dioxide (EMD), which has been improved so that, when it comes in contact with a metal material, corrosion of the metal material is reduced when compared to contact with conventional EMD. Moreover, certain embodiments of the invention are directed to methods for producing the improved EMD by applying a corrosion inhibitor to a neutralized EMD such that corrosion of a metal material arising from contact with the improved EMD is reduced. Furthermore, the invention relates to batteries including the improved EMD.
2. Description of the Related Art
EMD is commonly used as an active material for dry battery cells because it is an inexpensive and abundant material and it provides excellent discharge and long-term storage performance. For example, EMD is used as a material in the positive electrode of a primary alkaline or lithium battery or as a precursor for an active material of a positive electrode in a lithium-ion battery.
EMD is typically prepared by passing a direct current through an acidic solution of manganese sulfate and sulfuric acid. The positive electrode of this plating may include a plate of titanium onto which the EMD is deposited. The negative electrode may be made of graphite or copper, or similar material. The deposited EMD is mechanically removed from the titanium plate after it has reached a thickness of about 1 mm to about 75 mm. EMD pieces removed from the titanium plate are reduced in size to meet the requirements of the battery manufacturer using a grinding or milling process. The resulting material is referred to as milled EMD.
Because the EMD is prepared in an acidic bath, the preparation of the EMD generally requires a washing and/or caustic treatment of the EMD to neutralize the acidity of the bath. This treatment may be performed before or after the milling of the EMD. The resulting material after this neutralization step is referred to as neutralized EMD.
In a final step, the EMD is dried to certain specifications. For example, for primary alkaline-battery applications, the drying step is generally mild, leaving behind the chemically bound water and physisorbed water, which may range from about 1% to about 3% of the product weight. The resulting material is the active material for primary alkaline batteries.
For primary-lithium-battery applications, the active material is prepared by removing all water, whether chemical or physisorbed, to avoid any reaction with the organic electrolyte and/or the metallic lithium in the battery.
Because the EMD is prepared in a solution of manganese sulfate or sulfuric acid, there is a high concentration of sulfate associated with the material, typically 1-1.4% by weight of the material, after drying. Without being bound by corrosion mechanisms, sulfate is a known corrosion aggressor ion that will facilitate or promote the corrosion of iron found in a steel piece.
Battery manufacturers use the EMD as an active material of the positive electrode in alkaline cells against a zinc anode. The EMD is combined with other materials that make up the positive-electrode precursor, which is compacted by tools, frequently made from a metal material such as steel, before or during insertion of the mix into metal cans. These cans form the housing for batteries and at the same time act as the current collectors for the positive electrode. Because various parts of the production line involve metal tools, for example, tools made from steel that are in direct contact with the EMD, there is a risk of accelerated corrosion which leads to increased tool wear beyond normal expected mechanical wear. A particular concern is the dies and pins that are used to form hollow cylinders from the positive-electrode precursor. These cylinders are formed under high pressure, which reduces the useful life of the dies and pins caused by mechanical wear. If these parts are made of steel, sulfate may accelerate the corrosion of the steel, and then by erosion hasten the tool wear of these parts, shortening the life of the steel parts.
The current collectors for the positive electrode used by battery manufacturers are typically made from steel which is nickel coated on the surface in contact with the positive electrode. If this nickel coating is compromised during its formation, worn, scratched or too thin, the sulfate may initiate corrosion on the contact surface. Corrosion on the current collector in the battery has the potential of adversely affecting battery performance.